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Cold subduction of oceanic crust: Implications from a lawsonite eclogite from the Dominican Republic

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Zack, Thomas; Rivers, Toby; Brumm, Richard; Kronz, Andreas
European Journal of Mineralogy
Lawsonite eclogite is a rare rock type that has been described from only five natural occurrences. In contrast, laboratory experiments and thermal models predict that lawsonite eclogite should be widespread in subducted oceanic crust deeper than 1.5 GPa. Here we report a new lawsonite eclogite find from the Dominican Republic that provides constraints on the conditions of subducted crust and on its return to the surface. In this sample, lawsonite coexisting with omphacite occurs as both inclusions in garnet and as porphyroblasts, the latter being partly replaced at their margins by epidote and zoisite. Peak pressure conditions estimated from lawsonite-phengite-omphacite-garnet assemblages were <i>ca</i> 1.6 GPa at a temperature of 360°C, implying formation under a geotherm of <i>ca</i>. 8°C/km. Peak temperature conditions of 410-450°C were in the zoisite eclogite field, suggesting that the sample crossed from the stability field of lawsonite eclogite into that of zoisite eclogite as a result of increasing temperature. A comparison with other reported occurrences indicates that most lawsonite eclogite exhumed at the Earth's surface formed in accretionary wedges. The rarity of lawsonite eclogite at the Earth's surface may be principally due to two factors: (i) that in 'normal' subduction settings lawsonite eclogite enters the subduction factory and hence is usually not exhumed; and (ii) that in accretionary wedge settings, where the <i>PT</i> path leaves the stability field of lawsonite eclogite due to heating, lawsonite eclogite is only preserved if the exhumation path is constrained to a narrow window where the terminal stability of lawsonite is not crossed.
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